This invention relates to a combinatorial weighing method. More particularly, the invention relates to a combinatorial weighing method for a case where weighing is executed to obtain a target weight value greater than a permissible weight value, the method including steps of dividing the target weight value into a plurality of target values, executing combinatorial weighing for each resulting target value, and discharging the weighed articles on the basis of each of the combinations obtained as a result of the respective combinatorial weighing operations.
A combinatorial weighing method which is known in the art makes use of a plurality of weighing machines each consisting of a weighing hopper and a weight sensor associated with the weighing hopper. According to this known method, combinatorial weighing is carried out by weighing articles which have been introduced into the weighing hoppers of the weighing machines, selecting the combination of weighing machines (referred to as the "best" combination) that gives a total weight value equal to a target weight value or closest to the target weight value within preset allowable limits, discharging only those articles contained by the weighing hoppers of the selected weighing machines, subsequently replenishing the emptied weighing hoppers with new articles, and then finding the best combination again. The foregoing sequence of steps is repeated to carry out a continuous, highly accurate weighing operation in automatic fashion.
FIG. 1 is a schematic diagram of showing the mechanism of a combinatorial weighing apparatus for practicing the weighing method described above. Numeral 11 denotes a main feeder of vibratory conveyance type. Articles to be weighed are introduced into the main feeder 11 and imparted with vibratory motion for a predetermined length of time, so as to be dispersed radially outward from the center of the main feeder. Numerals 12, 12 . . . denote n-number of weighing stations which are arranged around the main feeder 11 along radially extending lines to receive the articles dispersed by the main feeder. Each weighing station 12 includes a dispersing feeder 12a, a pool hopper 12b, a pool hopper gate 12c, a weighing hopper 12d, a weight sensor 12e, and a weighing hopper gate 12f. The dispersing feeder 12a comprises an independently vibratable conveyance device for feeding the articles by means of vibration, or an independently operable shutter. In either case, each dispersing feeder 12a is so arranged that the articles received from the centrally located main feeder 11 can be introduced into the corresponding pool hopper 12b disposed therebelow. The pool hopper gate 12c is provided on each pool hopper 12b in such a manner that the articles received in the pool hopper 12b are released into the weighing hopper 12d when the pool hopper gate 12c is opened. Each weighing machine consists of a weighing hopper 12d and weight sensor 12e of its own, the latter being attached to the hopper 12d. The weight sensor 12e is operable to measure the weight of the articles introduced into the corresponding weighing hopper 12d, and to apply an electrical signal indicative of the measured weight to a combination control unit (not shown). The combination control unit then selects the combination of articles (known as the "best" combination) that gives a total weight equal to a target weight value or closest to the target weight value within preset allowable limits, as will be described below in further detail. Each weighing hopper 12d is provided with its own weighing hopper gate 12f. A drive control unit, not shown, upon receiving the signals from each of the weight sensors, produces a signal to open only the weighing hopper gates 12f of those weighing machines that give the best combination, these gates 12f discharging the articles into a common chute 13 where they are collected together. The collecting chute 13 has the shape of a funnel and is so arranged as to receive the articles from any of the circularly arrayed weighing hoppers 12d via the hopper gates 12f, which are located above the funnel substantially along its outer rim. The articles received by the collecting chute 13 are collected at the centrally located lower end thereof by falling under their own weight or by being forcibly shifted along the inclined wall of the funnel by a mechanical scraper or the like, which is not shown. The collecting chute 13 is provided with a timing hopper 14 at the lower end thereof for temporarily holding the collected articles. The arrival of an externally applied signal from a packaging machine or the like causes the timing hopper 14 to release the retained articles from the system.
In operation, articles are charged into each of the pool hoppers 12b and weighing hoppers 12d. The weight sensors 12e associated with the weighing hoppers 12d measure the weights of the articles and supply the combination control unit (not shown) with signals indicative of the measured weight values, denoted W.sub.1 through W.sub.n. The combination control unit computes combinations based on the weight values W.sub.1 through W.sub.n and selects the best combination of articles that gives a total weight closest to a target weight value. The drive control unit (not shown) responds by opening the prescribed weighing hopper gates 12f based on the best combination, whereby the articles giving said best combination are released into the collecting chute 13 from the corresponding weighing hoppers 12d. This will leave the selected weighing hoppers 12d empty. Subsequently, therefore, the pool hopper gates 12c corresponding to the empty weighing hoppers 12d are opened to introduce a fresh supply of the articles from the respective pool hoppers 12b into said weighing hoppers 12d, leaving these pool hoppers 12b empty. Accordingly, the dispersing feeders 12a which correspond to the empty pool hoppers 12b are vibrated for a predetermined period of time to deliver a fresh supply of the articles to these pool hoppers. This restores the weighing apparatus to the initial state to permit resumption of the control operation for selecting the best weight combinations in the manner described. Thus, weighing by the combinatorial weighing apparatus may proceed in continuous fashion by repeating the foregoing steps.
Let us examine a case where X.sub.a grams are to be weighed out in a combinatorial weighing apparatus having N-number of weighing machines. The target weight value will therefore be X.sub.a grams. To obtain the target weight, the amount of articles fed to each weighing machine should be adjusted to have an average value of 2X.sub.a /N grams when N is even, and 2X.sub.a /(N+1) grams or 2Xa/(N-1) grams when N is odd. The reason is as follows. The number of combinations that can be computed by a combinatorial weighing apparatus composed of N weighing machines, where a combination may be made up of only one weighing machine or up to all N of the weighing machines, is 2.sup.N -1. When N is even, combinations composed of N/2 weighing machines will be the largest in number among said 2.sup.N -1 combinations. When N is odd, combinations composed of (N+1)/2 or of (N-1)/2 weighing machines will be the largest in number among said 2.sup.N -1 combinations. For example, when N=10, only ten combinations made up of one weighing machine each will exist, whereas the number of combinations composed of five (i.e., N/2) weighing machines will be 252 in number. Accordingly, the weight of the articles fed into each weighing machine of the apparatus should be in the neighborhood of 1/(N/2) of the target value X.sub.a. In a case where the weight values of the individual article collections are controlled in this manner, there is a very high probability that the sought combination (i.e., the "best" combination) will exist in the combinations composed of N/2 or (N/2).+-. 0.5 weighing machines, thereby enabling an extremely accurate weighing operation. In other words, if we assume that the weight of the collection of articles fed into each weighing machine is approximately 2X/N grams, then the largest weight value (referred to hereinafter as the permissible weight value) that can be measured with great accuracy by the above-described combinatorial weighing apparatus will be X+.alpha. grams (O&lt;.alpha.&lt;X). In order to conduct a combinatorial weighing operation to obtain a target weight value X.sub.a greater than the largest permissible weight value, therefore, it is common practice to either (A) divide the target weight value X.sub.a into a number of weight values X1, X2, X3 . . . each of which is less than the largest permissible weight value, and then simply repeat the combinatorial weighing operation a plurality of times, or (B) divide the target weight into a number of weight values each of which is less than the permissible weight value and then, in conducting the weighing operation from the second weighing operation onward, correcting the target weights X2, X3, . . . that will prevail each time by the error resulting from the previous weighing operations.
Weighing method (B) outlined above will now be described in greater detail with reference to the flowchart of FIG. 2. We will assume that the target weight value X.sub.a is 3X grams, and that X1=X, X2=X, X3=X. In order to weigh out 3X grams of the articles, method (B) proceeds in the following fashion:
(1) First, all of the weighing machines are supplied with articles.
(2) The weights of the articles fed into the weighing hoppers of the weighing machines are measured (first weight measurement).
(3) Combinations are computed with X1 (=X) grams serving as the target, and the difference between X and Y1, which is the total weight value of the articles contained by those weighing machines that give the best combination, is stored in memory as an error E1 (=Y1-X).
(4) The articles are discharged from the weighing machines that give the best combination (first discharge operation).
(5) The emptied weighing hoppers of the weighing machines, that is, those that have discharged their articles, are supplied with articles afresh.
(6) The weights of the articles fed into the weighing hoppers of the weighing machines are measured (second weight measurement).
(7) Combinations are computed with X2 -E1 (=X-E1) grams serving as the target, and the difference between the target value (X-E1) and Y2, which is the total weight value of the articles contained by those weighing machines that give the best combination, is stored in memory as an error E2 (=Y1+Y2-2X). It should be noted that: ##EQU1##
(8) The articles are discharged from the weighing machines that give the best combination (second discharge operation).
(9) The weighing hoppers of the weighing machines that have discharged their articles are supplied with articles afresh.
(10) The weights of the articles fed into the weighing hoppers of the weighing machines are measured (third weight measurement).
(11) Combinations are computed with X3-E2 (=X-E2) grams serving as the target, and the articles are discharged from the weighing machines that give the best combination (third discharge operation). The end result is 3X grams of the articles.
Both of the methods (A) and (B) described above are attended by disadvantages. With method (A), combinatorial weighing is executed independently for each of the plurality of target values X1, X2, X3, as described. Consequently, since the batch of articles weighed out by each independent weighing operation may include an error, the errors resulting from the weighing operations would be compounded when the three weighed-out batches are combined into the single desired batch. The end result is poor weighing accuracy. For example, if the first combinatorial weighing operation generates an error of -10 grams and the second an error of -8 grams, then 982 grams of the articles would actually be weighed out for a target weight of 1000 grams.
With method (B), on the other hand, the operation of introducing articles into the weighing hoppers, measuring weights and then discharging the selected articles is repeated each time combinatorial weighing is performed for each and every one of the divided target values X1, X2, X3 . . . , making it impossible to carry out weighing at high speed because of the time involved for the weighing step. Since the weighing hoppers tend to oscillate whenever they are supplied with the articles, the length of the time required for repeated weighing operations can be particularly great since weight sensing cannot begin until the weighing hoppers stabilize.